Why do only circular rings are formed in fabry perot interferometer?

Question

I mean why not some other shape?

Answer 1

The nature of the Huygens wave front is that light disperses from its source as an ever expanding sphere given no constraints. So all these little wavelets that make up the wave front are expanding like a balloon expanding. When you use the interferometer, you are seeing a cross section of that balloon. The cross section of a sphere is a circle.

Answer 2

Because the etalon is usually driven from something that closely resembles a point source and the light radiates outwards along a spherical wavefront (Huygens' principle)


Doug